This invention is in the field of uninterruptible power supply systems in which a motor/generator pair is operated to provide a multi-phase output power signal in response to a power signal derived from a main power source, or from a standby DC source when the MAINS are interrupted.
In the uninterruptible power source (UPS) art, it is known to provide a synchronous motor which is coupled to a synchronous generator and to drive the motor by a power arrangement which derives energy from a multi-phase utility power signal. When the utility (MAINS) power signal is interrupted, a standby battery circuit connected to the motor driving arrangement provides the energy to drive the motor. The changeover from the MAINS to the standby source must be fast enough to result in no interruption of motor/generator operation, thereby ensuring that the multi-phase load power signal derived from the generator is "uninterruptible" and within the requirements of the critical load.
In the prior art, arrangements for driving the synchronous motor/generator pair include UPS systems which employ "offline" inverters. When the MAINS power drops, the offline inverter is switched on to convert the DC power of a standby battery circuit to a multi-phase power signal which drives the motor. The offline inverter characteristically employs high-power semiconductor switches, such as SCRs, for power conversion. When switchover occurs from MAINS to standby power, the SCRs must be switched on. When switchover occurs, it is not uncommon for a cold SCR to fail catastrophically when suddenly energized with a high-magnitude power surge.
Further, an UPS system utilizing an offline inverter must also employ an induction motor. At switchover, the rotation of a synchronous motor would not be in phase with the operation of the switched-on inverter without synchronization. Such synchronization is typically a synchronizing circuit which requires a significant amount of time to synchronize the inverter and the motor, during which time the output power signal could drop. At switchover, however, rotation of an induction motor does not need to be synchronized with the operation of the inverter. As is known, an induction motor exhibits "slippage", which is the difference between the speed of the induction motor, and the ideal motor speed required to give the correct voltage/frequency characteristic to the multi-phase power signal output by the generator. Since an induction motor characteristically rotates with a speed somewhat lower than that required for ideal generator operation, an UPS system employing an induction motor will give marginal operation due to the motor's slippage.
One prior UPS design employs a motor/generator pair in which both elements are synchronous, and which uses an inverter to provide a motor drive signal. The inverter is "quasi"-online, in that it is continuously provided a signal which keeps its power switching elements "on" throughout the operation of the UPS. However, while the MAINS source is operating as required, it alone drives the synchronous motor. Thus, at switchover, the operation of the inverter must be synchronized with the rotation of the synchronous motor. This requires the employment of synchronizing circuitry during switchover, which can result in degradation or interruption of multi-phase output power from the generator, if the synchronizing period is unduly long. Further, when the quasi-online UPS system is initially turned on, an auxiliary induction motor called a "pony" motor is employed to rotate the synchronous motor/generator combination. Once the synchronous motor/generator pair has been brought up to approximately synchronous speed, a drive signal for the synchronous motor, derived from the MAINS multi-phase power signal, is provided, and the auxiliary motor is turned off.